The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile.
In recent years, full-colorization of graphic information has been progressing at a remarkable pace, and with this progress, demand for an image forming apparatus capable of outputting a large number of beautiful hard copies, faster and less expensively, have been increasing.
In order to satisfy the above described demand, an electrophotographic image forming apparatus has been variously devised.
For example, in order to satisfy the demand for speed, that is, increase in image formation speed, some of the recent electrophotographic image forming apparatuses are provided with four image bearing members so that charging, exposing, and developing processes can be virtually simultaneously carried out for all primary colors, to achieve an image formation speed of four times that of a conventional image forming apparatus which comprises only a single image bearing member. This method of employing four image bearing members is called the four drum method.
From the standpoint of transfer medium, an image forming apparatus has been devised so that fixing speed can be varied to achieve high image quality regardless of the recording medium on which an image is formed. Generally, in order to achieve high image quality while forming an image on a cardboard, an OHP sheet, or the like, fixing speed is varied. More specifically, fixing speed is reduced with the use of a conveying belt dedicated for fixation, while keeping constant, or unchanged, the image formation speeds in the image formation processes prior to fixation process.
However, providing an image forming apparatus with such a structural arrangement tends to increase the size of the main assembly of an image forming apparatus. Thus, a structural arrangement which increases the speeds for all the image formation processes has been also proposed in addition to the above described structural arrangement. This second proposal makes it possible to eliminate the conveying belt dedicated for image fixation, making it therefore possible to reduce the size of the image forming apparatus main assembly. The size reduction leads to cost reduction. In other words, the second proposal makes it possible to provide a small and inexpensive image forming apparatus.
On the other hand, changing the image formation speed of an image forming apparatus requires changes in virtually entire aspects of the image forming apparatus, for example, the characteristics of the photoconductive member, characteristics of developing means, or characteristics of the transferring means. Consequently, it changes the characteristics of the image forming apparatus in terms of output.
For example, in the case of the photoconductive member characteristics, as the image formation speed is varied, the time it takes for a given point of the peripheral surface of the photoconductive drum to move from the exposing station to the developing station varies. Therefore, even if two image forming operations different in image formation speed are rendered equal in the amount of the exposure of a given point of the peripheral surface of a photoconductive member in the exposing station, the two image forming operations do not always become equal in the potential level of the corresponding points of the peripheral surface of the photoconductive member, and neither do the two image forming operations different in image formation speed always become the same in the characteristics of the developing means. These differences between the two image forming operations regarding the changes in the characteristics of the photoconductive member, developing means, transferring means, and the like, manifest as differences between the two image forming operations in terms of total image output.
Thus, in the past, an image forming apparatus has been structured so that it can be controlled to cancel the changes in the above described characteristics so that an image of good quality can be outputted. It is thought that the characteristics of an image forming apparatus in terms of image output subtly change due to the changes in the environmental conditions, such as the temperature and humidity of the ambience in which the image forming apparatus is operated.
The present invention relates to an image forming apparatus which has a plurality of image formation speeds, and the primary object of the present invention is to provide an image forming apparatus which has a plurality of image formation speeds, and is capable of always outputting an image of optimal quality at all of the plurality of image formation speeds of the image forming apparatus.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.